The role of cyclic nucleotides in growth control and differentiation is yet unresolved. The recent availability of forskolin, a drug which directly stimulates the catalytic subunit of adenylate cyclase, allowed to study growth regulation of brain cells by cAMP from a new viewpoint. Glial cells, endothelium and smooth muscle cells from brain microvessels were used for this in-vitro study. cAMP production, thymidine incorporation, and morphological and cytoskeletal changes were examined. The results demonstrated that in all three cell types, thymidine incorporation was reduced dose-dependently with maximal growth inhibition at 100 muM forskolin. A one hour preincubation with forskolin abolished thymidine incorporation in fetal serum (FCS) - containing medium over the following 24 hours. Exposure to forskolin led also to drastic and immediate changes of cell morphology and F-actin composition in endothelium and smooth muscle. These changes were reversible. In glial cells morphologic changes were visible only after exposure to forskolin for more than 24 hours. These changes were accompanied by increased staining with antibodies against glial fibrillary acidic protein. The findings support a role of cAMP in growth regulation of these cells and indicate that forskolin might be used as a tool to induce growth arrest and eventually differentiation in cell cultures from mammalian brain.